The measurement of sodium ion is of importance in several fields. In addition to important laboratory applications in chemistry and biology, the sodium ion electrode is finding acceptance in industry, particularly in water pollution and water purity applications. Power plants require very high purity water for successful operation of their generating equipment. Raw water must be extensively treated to remove impurities, and the high quality of water used in generating systems must be rigorously maintained.
Heretofore electrolytic conductivity measurements have been extensively used to monitor the quality of water in power plants. An electrolytic conductivity measurement has the limitation, however, of responding not only to the harmful impurities which operators want to detect but also to certain other substances they deliberately add to the boiler water to improve operating conditions. Recently, there has been a trend toward specific measurement of sodium ion to indicate the total impurity content of boiler water. Sodium ion is almost always present in substances that contaminate boiler water. Hence, the presence of sodium ion is a good indication of contamination, as may enter, for instance, through a leaky tube in a condenser unit.
Flame photometry has been used to measure the sodium ion content of boiler water. However, this measurement is rather difficult to make, requires trained personnel, and entails the use of complicated apparatus that is not suitable in its present state of development for continuous monitoring of plant streams. Accordingly, there has been considerable interest in attempting to adapt the sodium ion electrode to the problem.
Sodium ion analyzers of the electrode type are described in "Monitoring Sodium in High Purity Water," R. H. Jones, Industrial Water Engineering, Vol. 1, No. 2, March/April, 1964; and in Calgon Bulletin 5-1038-S, Sept. 1, 1965, Equipment and Instruments, a publication of the Calgon Corporation, Pittsburgh, Pennsylvania.
Sodium ion glass electrodes are similar to ordinary pH glass electrodes except that the composition of the glass used in the ion sensitive membrane is selected to respond primarily to the sodium ion rather than to the hydrogen ion as in the pH electrode. Whereas the pH electrode responds faithfully to hydrogen ion over an extremely wide range of concentration and almost without any interference effects, the sodium ion electrode will respond not only to sodium ion but also to hydrogen ion and to a number of other cations. By shifting the glass composition, one can modify the relative degree of response to these several cations, but thus far it has not been possible to make an electrode that is uniquely sensitive to sodium ion alone.
The discrimination capability of a sodium ion electrode is expressed by the so-called selectivity ratio, which is a measure of the extent to which the electrode will respond to two different ions in question, for example, sodium ion versus hydrogen ion, sodium ion versus potassium ion, or sodium ion versus ammonium ion. The selectivity ratio constant, K may be thought of as that factor by which the activity, or concentration, of a second ion species must be multiplied in order for the electrode to respond equally to the two species. Electrodes are usually very much more selective to hydrogen ion than to metal ions, and the K values (hydrogen/sodium) may range from 10 to 1000. Accordingly, for equal response to hydrogen ion and the metal ion, the hydrogen ion activity (concentration) must be much less than the metal ion activity value.
In some applications, the level of sodium ion may be sufficiently high that measurements can be made successfully in neutral (7pH) solutions, or even at lower pH values. In other applications, the level of sodium ion may be quite low, in which case the pH of the sample must be raised to a suitable value to eliminate effects of hydrogen ion on the measurement.
For example, in boiler water applications it is desired to measure very low levels of sodium ion, at times going down to 1/10 part per billion. This concentration corresponds to a molarity or normality of approximately 4 .times. 10.sup.-.sup.9 in sodium ion. In order to measure this very low concentration of sodium, the hydrogen ion concentration must be reduced.
The prior art has accomplished this by adding a buffering agent, or base, to the sample liquid in order to reduce hydrogen ion activity. Commonly, ammonia and morpholine have been added to the sample liquid to raise the pH so that the hydrogen ion background is reduced. These prior art techniques have not been completely successful, particularly where extremely low levels of sodium ion concentration must be measured. As noted in the article "Beckman's Specific Ion Electrodes," Beckman Analyzer, Vol. 4 (1) 1963, page 3, the pH of the resulting solution must be raised to a value higher than 10.5. This article attributes the adverse effect on the electrode during prolonged contact to the high pH or high hydroxyl ion content of the solution.